Nature is capable of engineering some pretty crazy stuff, so maybe it shouldn't come as a surprise that many of us are infected with a parasite that has quietly invaded our brain and is probably, as we speak, affecting things like how we drive, whether we like the smell of cat pee, how social we are, and even how we see the world. When you put it like that, it sounds insane. If that were actually true, certainly we would already know about it, right? Nope. But now we do.

What we do know so far about this batty-sounding theory is thanks largely to Jaroslav Flegr, a Czech scientist, who is the subject of a truly fascinating piece in the Atlantic by Kathleen McAuliffe. Flegr came to believe, over a period of years, that he was being controlled by a single-celled parasite called Toxoplasma gondii (T. gondii or Toxo). And when he says controlled he means his mind, as in the little bugger is in his brain, steering him to do strange and dangerous things. And if it was happening to him, he thought, maybe it was happening to lots of people. If that sounds crazy, that's because it is. But just because something is crazy doesn't mean it's not true. And, though his work with Toxo was on the margins of science for years, he's now gaining mainstream acceptance, and his research has been confirmed by any number of other highly reputable scientists. So, in other words, it looks like he's onto something.

So what exactly is Toxo? It's a parasite which is found in cat poop, and you might know it best because it causes toxoplasmosis. It's what pregnant women aren't supposed to catch, and the reason they're excused from cleaning the litter box. If they do catch it, it can transmit to the baby and result in brain damage and death. It can also infect those with compromised immune systems and cause dementia, but healthy people usually just get a quick flu-like bout with it, and then the Toxo is thought to go dormant and rest in your brain. Except it might not be dormant after all.

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Toxo's main purpose is to pass itself between cats—since that's the only place it can reproduce. So it comes out in cat shit, and from there it makes its way into grazing animals, and then, Flegr believes, it starts controlling their behavior in order to bring them back in contact with cats. Humans usually get the parasite from litter boxes, and via drinking water, unwashed veggies, or undercooked meat. It's thought that between 10 and 20 percent of Americans have Toxo lurking inside them.

The degree to which it might affect us is actually astonishing, but first let's talk about its effects on rats. For starters, normal rats are terrified of cat pee. (Who can blame them? It's foul.) But Toxo-infected rats are drawn to the smell of it. They're also much more active than non-infected rats, which could make them more attractive hunting targets for cats, and they're also "less wary of predators in exposed spaces." So basically they're an open invitation for a hungry kitty.

While the parasite might infect humans accidentally—since we're not a direct path back to a cat—that doesn't mean it doesn't still exert an effect on us. One of Flegr's original findings was that it delays people's reaction times, which proves very bad for their driving abilities. He found that those with the parasite were 2.5 times as likely to be in a car accident than those who didn't have it. Which could mean, according to Flegr, that it causes "several hundred thousand road deaths each year." Ahhh!

It also plays with our emotions, and Flegr found it causes many sex-specific personality changes:

Compared with uninfected men, males who had the parasite were more introverted, suspicious, oblivious to other people's opinions of them, and inclined to disregard rules. Infected women, on the other hand, presented in exactly the opposite way: they were more outgoing, trusting, image-conscious, and rule-abiding than uninfected women.

That is bananas. Oh, and infected men enjoy the smell of cat pee just like the rats do. Infected women, for whatever reason, find cat piss stench much more repellent than non-infected women.

One of the most serious and terrifying possible effects of Toxo is schizophrenia. Many schizophrenics show shrinkage in the cerebral cortex, and in a study done by Flegr and his colleagues, they gave 44 schizophrenic patients MRIs, and 12 of them had shrinkage. It turns out the ones who had it were almost exclusively the ones who also had Toxo. Cue the suspenseful music…

McAuliffe also talked to a psychiatrist named E. Fuller Torrey who says that despite popular claims that schizophrenia has always been around, that's not the case:

In fact, he says, schizophrenia did not rise in prevalence until the latter half of the 18th century, when for the first time people in Paris and London started keeping cats as pets. The so-called cat craze began among "poets and left-wing avant-garde Greenwich Village types," says Torrey, but the trend spread rapidly—and coinciding with that development, the incidence of schizophrenia soared.

DUN DUN DUN. But seriously it's hard not to wig out in the extreme with the thought that one of the most common mental illnesses could actually be brought on by a random parasite. And if that's not quite enough for you, you'll also want to know that there have been some studies that have linked Toxo infection to a higher risk of suicide, independent of mental illness. Great, just great.

So what exactly are we supposed to do with this information? Just sit back in a chaise lounge until the Toxo tells us to get up and walk into moving traffic? Umm, kind of, because there is no treatment. Once it has made a home in your brain, it's beyond the reach of antibiotics. There is some hope that anti-malarial drugs might work, but that's down the road. So for now, we're stuck with it.

As ridiculous as this sounds following all that, most of the scientists think there's actually no reason to panic. Famed Stanford neuroscientist Robert Sapolsky, who's become involved with some of Flegr's work, says "I'm not too worried, in that the effects on humans are not gigantic." He also thinks there are many more parasites that we're not even aware of acting on us (and other mammals) in other ways, which I guess is supposed to be comforting, but might also have the complete opposite effect on some of us.

If you want some scientific back-up for the wave of anxiety that is currently washing over you, Joanne Webster, a parasitologist at Imperial College London, is your gal:

I don't want to cause any panic. In the vast majority of people, there will be no ill effects, and those who are affected will mostly demonstrate subtle shifts of behavior. But in a small number of cases, [Toxo infection] may be linked to schizophrenia and other disturbances associated with altered dopamine levels—for example, obsessive-compulsive disorder, attention-deficit hyperactivity disorder, and mood disorders. The rat may live two or three years, while humans can be infected for many decades, which is why we may be seeing these severe side effects in people. We should be cautious of dismissing such a prevalent parasite.

I will just be over here moving my things into my new plastic bubble room, if anyone needs me…

OK, so the best thing to do is keep from getting Toxo in the first place. Fortunately for those of you who are deeply attached to your pet cats, you do not need to part company with them. Indoor cats don't have it, and outdoor cats only shed it for three weeks in their life ("typically when they're young and have just begun hunting"). During that time, you'll want to keep your kitchen extra clean.

More importantly, you always want to clean your vegetables carefully, drink purified drinking water, and ask for your meat well done. Done, done, and done! Other than that, you're free to go out into the world and pretend that you're operating of your own volition and not being controlled by a bunch of tiny parasites in your brain. A tinfoil hat is optional.